1. Field of the Invention
The present invention relates to a method of controlling the fuel injection pressure of an internal combustion engine common rail injection system.
The present invention may be used to particular advantage, though not exclusively, in automotive applications, to which the following description refers purely by way of example.
The present invention, in fact, may also be used in so-called xe2x80x9cno-roadxe2x80x9d, i.e., non-automotive, applications to control internal combustion engines of pumps, welders, generators, etc., and more generally in any industrial application requiring the generation of mechanical power.
2. Description of the Related Art
As is known, the common rail injection systems currently installed on vehicles comprise a number of injectors for drawing high-pressure fuel, under the control of an electronic control unit, from a common rail, and injecting it into respective cylinders.
The common rail is supplied with fuel by a high-pressure mechanical, normally piston, pump, in turn supplied with fuel from the vehicle tank by a low-pressure pump. The pressure of the fuel in the common rail is controlled by a pressure regulator, which drains any surplus fuel, pumped in excess of requirements, from the common rail to keep the common rail at a given pressure, during injection, depending on the power required.
The pressure regulator normally comprises a solenoid valve, i.e., a valve controlled by an electromagnet, which, when closed, allows supply to the common rail of all the fuel pumped by the high-pressure pump, and, when partly or fully open, drains the surplus fuel from the common rail along a drain conduit into the tank.
More specifically, the solenoid valve comprises a shutter, which is kept closed by a spring when the electromagnet is deenergized, and which is kept open when the electromagnet is energized. More specifically, the electromagnet is driven by the electronic control unit by means of a control signal, the duty cycle of which determines the extent to which the electromagnet is energized, and therefore the extent to which the shutter is opened.
Since, in common rail injection systems currently installed on vehicles, the high-pressure pump is a continuous-delivery pump not timed with the engine, i.e., a pump activated, for example, by a cam, to supply fuel substantially continuously to the common rail, whereas the injectors are activated at a given stroke in the engine cylinder cycle, the high-pressure pump must be designed to ensure maximum fuel draw by the injectors as a whole during the engine cycle.
European Patent Application 01130851.7xe2x80x94filed by the present applicant on 27, Dec. 2001, published on 3, Jul. 2002 under number EP-1219827, and claiming priority over Italian Patent Application TO2000A01228 of 29, Dec. 2000xe2x80x94recently proposed a special common rail injection system configuration which can also be fitted to old-type diesel engines, in which, as is known, the injectors are supplied directly by a high-pressure pump whose delivery is discontinuous, timed with the engine, and cyclically constant, i.e., a pump operated synchronously, i.e., pumping in time with the injectors.
More specifically, the above patent application describes a high-pressure pump comprising one or more pumping elements, each having a cylinder and a piston, which is activated by a corresponding cam to pump in time with the relative injector to appropriately control fuel pressure variations in the common rail. The cams are rotated by the engine, and more specifically are carried by a pump drive shaft preferably defined by an engine shaft performing other functions, such as the shaft operating the cylinder intake and exhaust valves, or the drive shaft itself.
In the case of a high-pressure pump with one pumping element, the piston is controlled by a cam shaped to produce a number of axial movements of the piston inside the cylinder, and so produce a number of fuel deliveries to the common rail at each engine cycle.
In some applications, the high-pressure pump piston is controlled by an asymmetrical cam, i.e., a cam shaped to produce a number of different axial movements of the piston inside the cylinder.
FIG. 1 shows an example of the movement of the high-pressure pump piston at each engine cycle as a function of the asymmetrical cam profile. More specifically, FIG. 1 relates to fuel supply to a two-cylinder engine by a high-pressure pump with one pumping element, and wherein two fuel injections, one per cylinder, are made at each engine cycle.
It should be pointed out that further injections per cylinder can be made by drawing directly from the fuel in the rail, which remains pressurized even in the absence of delivery.
As can be seen, during the engine cycle, the cam produces a first and a second axial movement of the piston inside the cylinder, to produce a first and a second fuel delivery to the common rail, so that the fuel in the rail is brought to a given required pressure value PR at the instants in which fuel is injected into the first and second cylinder respectively.
As shown in FIG. 1, a drawback of common rail injection systems of the above type lies in the fuel pressure in the common rail during the second delivery reaching, at the instant of synchronism, a different (typically higher) value with respect to the required pressure value PR due to dispersion.
The amount of fuel, in fact, in the high-pressure pump at the start of the second delivery is less than at the start of the first delivery, and the geometric effect of the reduction in volume inside the cylinder, as the piston advances along the cylinder, produces a greater increase in pressure at the second delivery, on account of the elastic fuel volumes on which compression is exerted being smaller than at the start of the first delivery.
This difference in fuel pressure in the common rail during deliveries after the first has various negative effects on both consumption and engine emissions.
It is an object of the present invention to provide a method of controlling the fuel injection pressure of an internal combustion engine common rail injection system, designed to eliminate the aforementioned drawbacks.
According to the present invention, there is provided a method of controlling the fuel injection pressure of an internal combustion engine common rail injection system, as claimed in claim 1.